Al Capone wasn’t prosecuted for tax evasion because it’s cool or smart or strategic to prosecute murderers for lesser crimes, but because proving murder in court was going to be more difficult.
That’s not analogous to a Congressional impeachment, but the opposite of how Congress operates. Congress sits on indisputable evidence of the greatest crimes while impeaching presidents for lesser offenses that are harder to prove.
Andrew Johnson publicly did everything he could to limit “freedom” for African Americans to a meaningless word. He was impeached for firing the Secretary of War.
Richard Nixon had indisputably bombed Cambodia, a crime that one failed article of impeachment charged him with, not to mention Vietnam and Laos. In fact, he had sabotaged the peace process and kept a war going for years during which millions of people were killed. Lyndon Johnson (who had committed similar horrors) believed Nixon guilty of treason for the sabotage. When Nixon fled Washington, he was about to be impeached for employing a group of thugs to break into a Democratic Party office.